How many times have we been led into the dungeons of guilt for our sleeping habits? After all our body's internal clock programs are often unique and some people just require more or less sleep than others. Challenging the conventional wisdom about the sleeping habits of our ancestors, scientists have now revealed that like us, they probably did not get eight hours of sleep a night either.

They stayed up late into the evening, averaging less than 6.5 hours of sleep and rarely took naps.

After studying indigenous ethnic groups like the Hadza of Tanzania, the San of Namibia and the Tsimane of Bolivia, authors suggest that the industrialised world's sleep habits do not differ much from those that humans evolved to have.

"The argument has always been that modern life has reduced our sleep time below the amount our ancestors got but our data indicates that this is a myth," said Jerome Siegel, lead researcher and professor of psychiatry at the University of California-Los Angeles.

"I feel a lot less insecure about my own sleep habits after having found the trends we see here," added another lead author Gandhi Yetish from the University of New Mexico.

During the biological night, we experience changes in hormone levels, body temperature and the propensity to sleep. The 8-hour sleep per night for optimum health is mostly a myth. "Somehow the clock programs an internal night during which it favors rest and sleep," said Aeschbach, who is now at Brigham and Women's Hospital and Harvard Medical School in Boston, Massachusetts. But those hours fluctuate depending on the person.

The findings do validate some common ideas about sleep and health, including the benefits of morning light, a cool bedroom and a consistent wake-up time.

To reach this conclusion, Siegel and his team started studying sleep among traditional peoples two years ago.

Researchers clocked sleep patterns among the Hadza, hunter-gatherers who live near the Serengeti National Park and the Tsimane, hunter-horticulturalists who live along the Andean foothills.

The team collected sleep records on 94 adults for a total of 1,165 days.

One myth dispelled by the results is that in earlier eras people went to bed at sundown. The subjects of the study stayed awake an average of three hours and 20 minutes after sunset.

"The fact that we all stay up hours after sunset is absolutely normal and does not appear to be a new development, although electric lights may have further extended this natural waking period," Siegel explained.

Most of the people slept less than seven hours each night, clocking an average of six hours and 25 minutes.

There are expectations that we should all be sleeping eight or nine hours a night and that if you took away modern technology people would be sleeping more. "But now for the first time we are showing that is not true," the authors noted.

There is no evidence that these sleep patterns took a toll on people's health.

In fact, extensive studies have found that these groups have lower levels of obesity, blood pressure and atherosclerosis than people in industrialised societies and higher levels of physical fitness.

They rarely took naps. "There's this myth that humans used to take daily naps, but that now because we are so busy, we suppress the naps," Siegel said.

"In fact, napping, is relatively rare in these groups".

Insomnia was so rare among those studied that the San and the Tsimane do not have a word for the disorder.

The team was surprised to find that all three groups receive their maximal light exposure in the morning.

This suggests that morning light may have the most important role in regulating mood and neurons that serve as the brain's clock.

Morning light is uniquely effective in treating depression. "Many of us may be suffering from the disruption of this ancient pattern," Siegel concluded in a paper published in the journal Current Biology.